Medium: water; 900 mL.

Apparatus 1: 100 rpm.

Time: 60 minutes.

Tolerances— Not less than 80% (Q) of the labeled amounts of butalbital (C11H16N2O3), caffeine (C8H10N4O2), and aspirin (C9H8O4) is dissolved in 60 minutes.

Solvent— Add 1 mL of phosphoric acid to 1000 mL of methanol, and mix.

Standard preparation— Prepare a solution of USP Salicylic Acid RS in Solvent having a known concentration of about 0.0012 mg per mL. Use this solution promptly.

Test preparation— [NOTE—Perform this test on the same day the tablets are powdered.] Weigh and finely powder not fewer than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 65 mg of aspirin, to a 200-mL flask, add 100.0 mL of Solvent, and shake by mechanical means for 15 minutes. Filter a portion of this solution, discarding the first 15 mL of the filtrate, and use the clear filtrate as the Test preparation. Use this solution within 20 minutes after the addition of the Solvent.

Procedure— Immediately place the cell containing the solution under test in the cell compartment of a spectrophotofluorimeter, and allow to equilibrate for 2 minutes. Concomitantly measure the intensities of the fluorescence of the Test preparation and the Standard preparation at 444 nm, using an excitation wavelength of 305 nm. Calculate the percentage of salicylic acid in the portion of Tablets taken by the formula: in which C is the concentration, in mg per mL, of USP Salicylic Acid RS in the Standard preparation; a is the quantity, in mg, of aspirin in the portion of Tablets taken to prepare the Test preparation, based on the labeled amount; and IU and IS are the fluorescence intensity readings obtained from the Test preparation and the Standard preparation, respectively. [NOTE—If the intensity of the Test preparation greatly exceeds that of the Standard preparation, immediately transfer 5.0 mL of the Test preparation to a 50-mL volumetric flask, dilute with Solvent to volume, and mix. Immediately determine the intensity of this solution, and calculate the percentage of salicylic acid in the portion of Tablets taken by the formula: ] Not more than 3.0% is found.

(Official January 1, 2007)

0.01 M Phosphate buffer, Mobile phase, pH 2.5 Buffer, Aspirin standard stock solution, Standard preparation, Salicylic acid solution, and Chromatographic system— Proceed as directed in the Assay under Butalbital, Aspirin, and Caffeine Capsules.

Assay preparation— Weigh and finely powder not fewer than 20 Tablets. Transfer an accurately weighed portion of the powder, equivalent to about 325 mg of aspirin, to a 200-mL volumetric flask, dilute with pH 2.5 buffer to volume, and sonicate for 30 minutes. Pass a portion of this solution through a suitable filter of 0.5 µm or finer porosity, and use the filtrate as the Assay preparation. Use this solution within 24 hours.

Procedure— Separately inject equal volumes (about 10 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, using the 277-nm detector to record the caffeine and aspirin peak areas and the 210-nm detector to record the butalbital peak areas, and measure the peak areas for the major peaks. Calculate the quantities, in mg, of butalbital (C11H16N2O3), aspirin (C9H8O4), and caffeine (C8H10 N4O2) in the portion of Tablets taken by the same formula: in which C is the concentration, in mg per mL, of the appropriate USP Reference Standard in the Standard preparation; and rU and rS are the peak responses of the corresponding analyte obtained from the Assay preparation and the Standard preparation, respectively. Correct the amount of aspirin obtained for the amount of salicylic acid present taken by the formula: in which A is the quantity, in mg, of aspirin in the portion of Tablets taken to prepare the Assay preparation; and F is the percentage of salicylic acid obtained in the test for Limit of free salicylic acid.